Let us now consider the perturbative computation of the mean values (REF ). The perturbative expansion [1]}, [2]} of a generic expectation value (REF ) can be written as
\(\left\langle {\cal P} [\phi ] \right\rangle = \frac{ \left\langle \, {\cal P} [\phi ] \, e^{i S_I [\phi ]} \, \right\rangle _0 }{\left\langle \, e^{i S_I [\phi ] } \, \right\rangle _0 } \; ,\) 
